What is the difference between same convolution and full convolution in terms of feature map size?
What happens to the size of output feature map in case of full convolution?

It increases.

First one is valid padding: the blue square is not padded, so the green square is smaller.  Third one is same padding: the blue square is padded just enough so that the green square is the same size.  Fourth one is full padding: the blue square is padded as much as possible for that size of filter, so the green square is larger.
From here.